This invention relates to the production of mirrors, and, more particularly, to a powder metallurgical technique for manufacturing large numbers of mirrors having multiple reflective surfaces.
Common plate glass mirrors are found in nearly all homes, and are familiar to most persons. To prepare such mirrors, a very smooth glass surface is formed, and then a reflective coating is deposited over the smooth glass surface. The reflective coating is instrumental in achieving a high degree of reflectance, but the glass surface itself must be smooth so that the image is not distorted and possibly lost in part because of stray reflections. The preparation of glass mirrors benefits greatly from the availability of a process for making very smooth glass surfaces, and the design of the mirror in which there is only one reflecting surface.
Other types of mirrors are prepared in a similar fashion, wherein a very smooth surface is coated with a reflecting coating. In some instances, the preparation of smooth, optical quality reflective surfaces is quite difficult, and presents a major obstacle to the manufacturing of the mirror. Once the reflective surfaces are prepared, coating is relatively simple.
To cite a particular example of interest, rotating mirrors are used in some types of imaging systems. Such mirrors are toroids, or donut shaped, with a large number of either inwardly or outwardly facing reflective faces positioned on the inside or outside of the donut, respectively. The reflective faces are positioned at a variety of angles to reflect only one portion of an image to a detector at any moment. As the mirror rotates about the toroid axis, the reflective faces serially decompose the image so that it can be serially analyzed by the detector, transmitted electronically, signal processed, and finally reconstructed elsewhere if necessary.
In one increasingly familiar example, some laser bar code reading systems in supermarkets employ a rotating toroidal mirror having a large number of outwardly facing reflective facets. The facets are typically flat surfaces that are intentionally oriented between 6 and 18 degrees from the toroid axis. Light from an image is decomposed by the mirror and transmitted to a sensor, which reads the bar codes. This type of system is required because the bar codes may be presented at any angle and may be misoriented.
For many applications, such multifaceted mirrors must be made of metal, rather than glass. The mirror may be spun about its axis at a rate of up to 3600 revolutions per minute, requiring high strength and resistance to failure. Glass is too unreliable a material of construction for such a use. Making the mirror of metal permits it to be balanced readily, an important requirement when the mirror is to be spun rapidly. Moreover, no method of fabrication is now known to make such mirrors of glass in a highly precise toroidal form with a large number of internal facets, and with high perfection at the intersection lines of the facets.
Multifaceted metal mirrors have in the past been manufactured by one of two methods. In one, a metallic structure of the correct shape is machined, and a number of separately prepared glass mirrors are bonded to the prepared metal surfaces. The mirrors produced by this approach are unreliable, because of the possibility of failure of the glass or the bond between the metal and the glass, particularly during temperature excursions. The success rate of preparation is small, typically producing 1 good toroidal mirror for each 50 attempted, because much of the mirror preparation is a handwork process that depends upon the skill and patience of the assembler. Such mirrors are therefore very expensive.
In another approach, the mirror is made entirely out of metal. The all-metal mirror is fabricated by first machining the general shape of the mirror, including the mirror facets, from metal bar stock. The facets are fine machined using diamond cutting tools in precision machinery that is mounted on granite bases and operated in a temperature-controlled environment. The final machining is accomplished on each of the mirror facets, with the intent that each facet be an optical quality surface with no scratches or irregularities. The machined facet surfaces are finally coated with gold or a similar metallic coating.
As may be appreciated, this machining approach is slow, and utilizes expensive machinery. The quality of the finished mirrors is sometimes low, unless extreme care and time are taken. Low mirror quality typically results from one of two sources. First, the optical-quality machining of a mirror facet may leave fine scratches, due to machinery irregularities or because of metallurgical irregularities in the metal being machined. Second, even if the mirror faces themselves are optical quality, the intersections between the facets may have irregularities. The intersection line of two facets is the line along which the two planar faces join. When using machine tools, it is difficult to maintain this intersection line perfectly straight, and without irregularities. Imperfections result in a scattering of light at the intersection lines termed stray light radiation. Such stray light effects significantly degrade the image quality of the decomposed, or decomposed and reconstructed, image. By the nature of machining operations, even those where care is taken, the optical quality machining of each intersection presents a new opportunity for creation of an imperfection. That is, machining operations are inherently of low reproducibility, where an extremely high degree of perfection is required. As a result of these various problems, for many manufacturers the success rate for production of such high-quality rotating metal mirrors is about 1 acceptable mirror for each 10 attempted. The cost of the mirrors is therefore high.
There therefore exists a need for a process of manufacturing multifaceted, all-metal mirrors having a number of optical quality reflective surfaces, and the mirrors made thereby. The process should produce mirrors of high quality with low cost. The present invention fulfills this need, and further provides related advantages.